Although some observers thought it was a one-year fluke, the so-called gender gap reappeared last year during congressional elections. Women voted an average 6 percent more Democratic than men in House races, according to some polls. In districts where there was no incumbent officeholder, the gap grew to 16 percent, according to a recent University of Michigan study.

White House political advisers are publicly airing concerns about Mr. Reagan's and the GOP's popularity lag among women. When the President's official liaison with women was reported to be a nonbeliever in the gender gap, she was quickly overridden. The administration formed a high-level council to deal with women's concerns.

Meanwhile, Democrats have been taken by surprise by the sudden windfall of female support. While the GOP makes headlines by acknowledging the existence of the women's voting bloc, Democrats have remained in the background.

''It's there for the Democrats to go after,'' pollster Louis Harris said last week of the women's vote. ''They haven't shown much interest in going after it.''

''We've got a long way to go,'' concedes Ann F. Lewis, political director for the Democratic National Committee (DNC). But she predicts that her party will have projects for the 1984 elections to ''maximize'' the gender gap. ''We will know even more about it in '84,'' she says. As one beginning, the DNC has slated a ''women's agenda day'' in Washington on Friday to hear the views of women's groups.

Democratic presidential contenders are also reaching out, putting women in key campaign posts and assigning staffers to women's issues. Candidate Walter F. Mondale has formed a ''women's working group'' to advise his campaign. John Glenn has met with women leaders, and an aide notes that in a recent poll Mr. Glenn's popularity was higher than Mr. Mondale's among women.

''Our approach to the gender gap is that it's an economic thing,'' says Kathy Buskin, press secretary to candidate Gary Hart. ''They vote differently because they've got less in their wallets.'' Mr. Hart has recently begun speaking out for pay equity for women.

Alan Cranston, whose arms control stand jibes with the views of many women, has won praise for his history of backing legislation on child care, domestic violence, and social security reform. The presidential candidate, like his other Democratic competitors, is conferring with women leaders.

Most of the Democratic field will have a chance to compete July 10 at the National Women's Political Caucus convention in San Antonio. ''We plan to draw distinctions'' among the candidates, says Kathy Wilson, president of the caucus, on ''just how far they will go to see that an Equal Rights Amendment is ratified , what are their plans for economic equity for women,'' and other issues.

Mr. Mondale has caused a minor flap by announcing he will skip the meeting because of a family vacation. ''It is one of those very, very, very tough calls, '' says Maxine Isaacs, his press secretary.

Despite early efforts by Democratic candidates, Washington pollster Dotty Lynch says, ''In a number of cases, the Republicans have paid more attention than the Democrats.''

In its effort to patch up the gender gap, the GOP has launched a series of Republican Women's Leadership Forums to discuss issues such as the economy and defense. The party will stress recruitment of women to run for office. But the main thrust of the effort, according to the Republican National Committee (RNC), will be better public relations.

While experts have been puzzling for the past two years about what issues move women voters, a University of Michigan study offers some answers.

Women, more than men, share a dislike of the arms buildup, support for ''humanitarian domestic policies,'' and pessimism about the economy, says the study. It concludes women's rights issues rank lower in importance, but help explain the gender gap among single and divorced people.